1. Field of the Invention
This invention relates to the use of ZSM-22 zeolite in aromatic hydrocarbon conversion reactions which produce dialkylbenzene compound product mixtures wherein the 1,4-dialkylbenzene or para isomer content is substantially in excess of its normal equilibrium concentration.
2. Description of Related Art
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the OIL AND GAS JOLRNAL, Vol. 69, No. 48(1971).
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.
In these prior art processes, the dimethylbenzene product produced has the equilibrium composition of approximately 24 percent of 1,4-, 54 percent of 1,3- and 22 percent of 1,2-isomer. Of the dimethylbenzene isomers, 1,3-dimethylbenzene is normally the least desired product, with 1,2- and 1,4-dimethylbenzene being the more useful products. 1,4-Dimethylbenzene is of particular value, being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers, such as "Dacron". Mixtures of dimethylbenzene isomers, either alone or in further admixture with ethylbenzene, have previously been separated by expensive super-fractionation and multistage refrigeration steps. Such process, as will be realized, involves high operation costs and has a limited yield.
Various modified zeolite catalysts have been developed to alkylate or disproportionate toluene with a greater or lesser degree of selectivity to 1,4-dimethylbenzene isomer. Hence, U.S. Pat. Nos. 3,972,832, 4,034,053, 4,128,592 and 4,137,195 disclose particular zeolite catalysts which have been treated with compounds of phosphorus and/or magnesium. Boron-containing zeolites are disclosed in U.S. Pat. No. 4,067,920 and antimony-containing zeolites in U.S. Pat. No. 3,979,472. Similarly, U.S. Pat. Nos. 3,965,208 and 4,117,026 disclose other modified zeolites useful for shape selective reactions. It is also known to incorporate various metals into zeolite catalysts by means of ion exchange.
The separation of disubstituted meta or ortho isomers from para disubstituted aromatic compounds is also a necessary but difficult step in the production of meta or ortho isomers. The differences in boiling points of some disubstituted aromatic compounds are so small that the use of distillation columns to separate the isomers is often commercially unfeasible. For example, the boiling point, at atmospheric pressure, of 1-isopropyl-2-methyl-benzene (ortho-cymene) is 178.3.degree. C., while that of 1-isopropyl-4-methylbenzene (para-cymene) is 177.1.degree. C. Accordingly, in the past techniques other than fractional distillation were used to separate the para-isomer from the mixture thereof with the ortho- and meta-isomers. For example, Young, U.S. Pat. No. 4,181,811, discloses a process for selectively cracking, in the presence of a catalyst, e.g., ZSM-5 zeolite, 1,4-disubstituted aromatic compounds from a mixture thereof with 1,2-disubstituted aromatic compounds.